This invention relates to a process for the preparation of 2,5-dimethyl-2,4-hexadiene in the liquid phase starting from 2,5-dimethyl-2,5-hexanediol.
2,5-dimethyl-2,4-hexadiene is a product of great importance since it is an intermediate in the synthesis of pyrethroids.
A number of synthesizing methods for 2,5-dimethyl-2,4-hexadiene are known. Among these, the following can be mentioned:
(1) From 2,5-dibromo-2,5-dimethyl hexane, by elimination of hydrobromic acid; PA0 (2) From 2,2,5,5-tetramethyl tetrahydrofuran, by heating in a sealed tube at 180.degree. C.-190.degree. C. with five times as much of 1% HCl; PA0 (3) By isomerizing 2,5-dimethyl-1,5-hexadiene in the presence of alcoholic solutions of alkalies at 180.degree. C.; PA0 (b 4) From 1-bromo-2-methyl-1-propane, in the presence of sodium in benzene at 55.degree. C.-65.degree. C.; PA0 (5) By isomerizing 2,5-dimethyl-1,5-hexadiene on alumina at 365.degree. C. or on mixtures of Cr.sub.2 O.sub.3 -Al.sub.2 O.sub.3 at 250.degree. C.; PA0 (6) From 2,5-dimethyl-1,5-hexadiene by heating to a boil with 4-toluene-sulphonic acid.
These methods are quite peculiar, have scanty importance from the commercial standpoint and are mainly adapted to laboratory-scale preparations.
There are also methods for preparing 2,5-dimethyl-2,4-hexadiene starting from 2,5-dimethyl-2,5-hexanediol (DHAD), which is the same starting compound as used in the method of the present invention.
Said methods, however, are cumbersome, expensive and poorly selective and comprise flowing DHAD over catalytic beds of alumina impregnated with orthophosporic acid, or deposted on alumina or admixtures of alumina and chromium trioxide, at temperatures of from 200.degree. C. to 300.degree. C.
Under such conditions, there can be formed, besides 2,5-dimethyl-2,4-hexadiene, other, undesirable, C.sub.8 H.sub.14 isomers.
It is known, on the other hand, (see HOUBENWEYL, Methoden der organischen Chemie, Vol. VI/3, pages 528-535) than diols having the two hydroxyls separated by four carbon atoms, such as 2,5-dimethyl-2,5-hexanediol, easily form tetrahydrofurans by intramolecular cyclization with elimination of water.
More particularly, formation is disclosed in the literature (see W. REPPE, Annalen der Chemie, Vol. 596, page 110, (1955) ) of 2,2,5,5-tetramethyl tetrahydrofuran by treating DHAD with aqueous solutions of phosphoric acid or potassium bisulphate under reflux conditions, or also with dilute sulfuric acid.